Refactor keytime/metadata and wallet encryption bugfix

Refactor keytime:

• Key metadata is kept in a CWallet::mapKeyMetadata (std::map<CKeyId,CKeyMetadata>).
• When generating a new key, time is put in that map, and new key is written.
• AddKeyPubKey and AddCryptedKey do not take a creation time argument, but instead pull it from that map, if it exists there.

Bugfix:

• AddKeyPubKey and AddCryptedKey in CWallet didn't override the CKeyStore definition anymore. This is fixed, as they no longed need the nCreationTime argument now.

Also a few related other changes:

• Metadata can be overwritten.
• Only GenerateNewKey calls GetTime(), as it's the only place where we know for sure a key was not constructed earlier.
• When the nTimeFirstKey is known to be inaccurate, it is set to the value 1 (instead of 0, which would mean unknown).
• Use CPubKey instead of std::vector<unsigned char> where possible.